The invention relates to a process for the preparation of crosslinked spherical polymers having high swellability and low contents of soluble fractions using peroxyesters as polymerization initiators.
Ion exchangers having as uniform a particle size as possible (referred to below as "monodisperse") have very recently become increasingly important because economic advantages can be obtained in many applications due to the more advantageous hydrodynamic properties of an exchanger bed of monodisperse ion exchangers. Monodisperse ion exchangers can be obtained by functionalization of monodisperse bead polymers.
One possible method for preparing monodisperse bead polymers is the so-called seed/feed process, in which a monodisperse polymer ("seed") is swollen in the monomer and then polymerized. Seed/feed processes are described, for example, in European Patent Applications 98,130 and 101,943
Seed polymers should have a high swelling index so that they are capable of absorbing a large amount of the added monomer in the seed/feed process. The swelling index ("SI") is defined as the quotient of the volume of the swollen polymer and the volume of the polymer that is not swollen. The swelling index can be controlled in a known manner by the content of crosslinking agent: low contents of crosslinking agent lead to high swelling indices and vice versa. Thus, for example, styrene polymers that are crosslinked with 0.8 to 2.0% by weight of divinylbenzene have swelling indices of 2.5 to 8 in toluene. However, seed polymers having a low degree of crosslinking have a very high fraction of uncrosslinked, soluble polymers. This fraction of uncrosslinked soluble polymers in the seed polymer is undesired in many respects:
1. The polymerization of the swollen seed can be impaired because the polymer fractions dissolved from the seed by the added monomer cause the particles to adhere to one another. PA0 2. The functionalization for the preparation of the ion exchangers can be complicated because the dissolved-out polymer fractions accumulate in the reaction solutions used for the functionalization. PA0 3. The end products (i.e., ion exchangers) may contain large amounts of soluble polymers, which may lead to undesired bleaching of the ion exchangers. PA0 (a) 96.5 to 99.0% by weight (preferably 97.3 to 99.0% by weight) of a monomer having one C.dbd.C double bond capable of free radical polymerization per molecule, PA0 (b) more than 0.8% by weight to not more than 2.5% by weight (preferably more than 0.8% by weight to 1.5% by weight, particularly preferably 0.801% by weight to 1.5% by weight, very particularly preferably 0.801% by weight to 1.0% by weight, especially preferably 1.0% by weight) of a crosslinking agent containing two or more (preferably two to four) double bonds capable of free radical polymerization per molecule, and PA0 (c) 0.2 to 1.0% by weight of at least one aliphatic peroxyester as polymerization initiator, PA0 R.sup.1 denotes an alkyl radical having 2 to 20 carbon atoms or a cycloalkyl radical having up to 20 carbon atoms, PA0 R.sup.2 denotes a branched alkyl radical having 4 to 12 carbon atoms, and PA0 L denotes an alkylene radical having 2 to 20 carbon atoms or a cycloalkylene radical having up to 20 carbon atoms.
The object of the present invention is to provide crosslinked spherical polymers having high swellability and low contents of soluble fractions.
It has now been found that polymers having low contents of crosslinking agent and correspondingly high swellability have particularly low contents of soluble polymer fractions when peroxyesters are used as initiators in their preparation.